Generally, a torque converter mainly comprises a pump, a turbine, and a stator provided therebetween, and is filled with a working oil, so that its eddy flows transmit a rotating force from the pump to the turbine.
When a ratio of rotational speed of the turbine to the pump is smaller, curved vanes of the stator change the direction of the working oil into a helping direction of rotation of the pump. Therefore, a ratio of torque of the turbine to that of the engine (the torque ratio) is increased by an assist of the stator.
On the contrary, the torque ratio, when the rotational speed ratio becomes larger, is reduced. When the rotational speed ratio exceeds, e.g., 0.8 through 0.85, the torque ratio decreases to 1 or less, thus deteriorating the efficiency of power transmission.
To remedy the above, a lockup control device has heretofore been proposed which comprises a lockup clutch incorporated in the torque converter, so that when the number of rotations of an input shaft connecting the turbine increases to enter into an area of the rotational speed ratio where the torque ratio is 1 or less, the lockup clutch is engaged to connect the pump directly with the turbine, and when in the area where the torque ratio is 1 or more, the clutch is disengaged to release the direct connection.
The conventional lockup control device, for example, detects car speed to engage or disengage the lockup clutch, but is unable to perform linear control with respect to a change of the rotational speed ratio. For example, even when a low car speed and a small number of rotations of the engine keep the torque ratio less than 1, no lockup control is carried out, thereby deteriorating the efficiency of power transmission which leads to a loss of efficiency and an increase in fuel expense.